So called proof of performance testing in a cable television or CATV system includes what is sometimes known as signal validation and leakage detection. CATV systems deliver RF (radio frequency) television signals to subscribers or viewers by means of coaxial cable. In the past, conflict arose between CATV operators and the aviation industry because many of the frequencies used by the CATV operators to carry television signals are also used by the aviation industry. Conventional coaxial cable distribution systems use the 55 MHz to 1 GHz frequency spectrum for forward path signals and under 55 MHz for return path signals. When signals leak from the CATV distribution system, the signals can interfere with aviation signals if the leaked signals are of sufficient strength. For this reason, CATV operators are required by the FCC to test their CATV systems upon installation and periodically to ensure that any leakage that occurs is within regulated limits. Such testing is outlined and required by 65 CFR 76.601 et seq., “Subpart K-Technical Standards”, incorporated herein by reference.
During testing, a pre-selector filter is used to improve carrier noise ratios by removing out of band interference and noise. Pre-selector filters are one or more of the filter types listed in Table A.
TABLE AFilter TypeEffectBandpassPasses only a desired segment ofband and rejects frequencies aboveand below the desired passband.LowpassPasses from a certain frequency andlower, while rejecting unwantedcarriers above the certain frequency.HighpassPasses from a certain frequency andhigher, while rejecting unwantedcarriers below the certain frequency.NotchRemoves a very small segment ofband while passing frequenciesabove and below the band.BandstopRemoves a larger segment of bandthan a notch filter and passesfrequencies above and below theband.
The first testing procedure consisted of carrying a wide range of fixed filters to the test site and connecting each filter sequentially to test the CATV system over the required frequency ranges. The disadvantages inherent in this procedure led to other testing devices.
A known tunable RF preselector used to test both the forward and return CATV paths is the VF-5 from Trilithic, Inc., Indianapolis, Ind. which tests the forward path using four tunable filters which cover four bands in the 55-880 MHz range and tests the return path using six fixed channel filters, T7 through T12. Although the device is portable, it is still bulky because of the electronics required for the tunable filters.
A known automated preselector filter is the DCVF-5, also from Trilithic, Inc., Indianapolis, Ind. Based on the manual preselector VF-5, the DCVF-5 offers automatic switching signal paths through T7-T13 fixed filters and tuning the 55-800 MHz variable filter bank under GPIB and front panel user control. The device is 17″ wide by 10.5″ high by 20″ deep, and the weight of the device is 45 lbs.
Neither of these preselector filters are truly convenient to use due to the bulk and weight of the devices.